add list of contributors for releases

I'm a hired gun here. :-)

@szabgab introduced a x_contributors key in the release's metadata. That patch inserts the list of contributors (if there are any) below the author.

if you want to have examples of distributions that have contributors, have a gander at Test-Strict and /Dist-Zilla-PluginBundle-DAGOLDEN

Two points:

* The list of contributors is folded by default, as I don't want to push the list of dependencies outside of the viewport by default.

* I rejuggled a little bit the author's div. I'm fairly sure I didn't break anything, but please, don't trust me, double-check. :-)